As an emission type electronic displaying device, an electroluminescence device (ELD) is known. Elements constituting the ELD include an inorganic electroluminescent element and an organic electroluminescent element (hereinafter referred to also as an organic EL element). Inorganic electroluminescent element has been used for a plane light source, however, a high voltage alternating current has been required to drive the element.
An organic EL element has a structure in which a light emitting layer containing a light emitting compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emitting layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of several volts to several tens of volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
A practical organic EL element is required to emit light of high luminance with high efficiency at a lower power. For example, disclosed are an element exhibiting higher luminance of emitting light with longer life in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives doped with a slight amount of a fluorescent compound are employed (see Patent Document 1), an element which has an organic light emitting layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (see Patent Document 2), and an element which has an organic light emitting layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (see Patent Document 3).
When light emitted through excited singlet state is used in the element disclosed in the above Patent documents, the upper limit of the external quantum efficiency (□ext) is considered to be at most 5%, because the generation probability of excited species capable of emitting light is 25%, since the generation ratio of singlet excited species to triplet excited species is 1:3, and further, external light luminous efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Princeton University (see Non-Patent Document 1), studies on materials emitting phosphorescence at room temperature have been actively carried out (see Non-Patent Document 2 and Patent Document 4).
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the luminous efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits almost the same performance as a cold cathode tube, and can be applied to illumination.
For example, many kinds of heavy metal complexes such as iridium complexes have been synthesized and studied (see Non-Patent Document 3).
An example employing tris(2-phenylpyridine)iridium as a dopant has also been studied (see Non-Patent Document 2).
Other examples of a dopant which has been studied include, L2Ir (acac) (L represents a bidentate ligand, acac represents acetylacetone) such as (ppy)2Ir (acac) (see Non-Patent Document 4), tris(2-p-tolylpyridine)iridium {Ir(ptpy)3}, tris(benzo-[h]-quinoline) iridium {Ir(bzq)3} and Ir(bzq)2ClP (Bu)3 (see Non-Patent Document 5).
A hole transport material is used as a host of a phosphorescent compound in order to increase luminous efficiency (see Non-Patent Document 6).
Various kinds of electron transport materials doped with a new iridium complex are used as a host of a phosphorescent compound (see Non-Patent Document 4). High luminous efficiency has been obtained by incorporating a hole blocking layer (see Non-Patent Document 5).
As for a green light emission, an external quantum efficiency of around 20%, which is a theoretical threshold, has been obtained, however, there has been a problem in that the efficiency is notably degraded when a high luminance of light is emitted. As for other colors of light, fully enough efficiency has not been obtained, and improvement has been desired. As an example of an organic electroluminescent element emitting blue light with a high efficiency, Patent Document 5 is cited. Further, as an organic electroluminescent element for practical application in the future, desired is an organic electroluminescent element which emits light with a high efficiency and a low power consumption.
As for a phosphorescent dopant used in an organic electroluminescent element, many examples have been disclosed, for example, in Patent Documents 6, 7, 8, 9, 10 and 11. Many of these disclosures attempt to attain high efficiency of emission, high color purity and high durability, however, none of currently known phosphorescent dopant for an organic electroluminescent element exhibits fully satisfactory properties, and further improvement is desired.
On the other hand, in the fabrication of a large screen organic electroluminescent element device using low molecular weight compounds, the generally used vacuum deposition method has been known to have a problem in facilities and energy consumption, and desired have been printing methods, for example, ink-jet printing or screen-printing, or coating methods, for example, spin coating or cast coating. In the fabrication of white light emitting element, a plurality of light emitting compounds having different maximum emission wavelengths are incorporated in the emission layer, however, it is rather difficult to deposit a plurality of compounds in the same ratio in every fabrication using a vacuum deposition method, suggesting a lower yield ratio in the organic electroluminescent element production. However, when a production process of an organic electroluminescent element employing, for example, the above printing methods or coating methods using materials soluble in a solvent becomes possible, by using a solution containing the same ratio of phosphorescent dopants, the same ratio of the dopants are incorporated in each organic electroluminescent element, and white light emitting organic electroluminescent elements emitting the same color can be stably produced.
For example, known are polymer illuminants such as a polyphenylene vinylene derivative (PPV) or a polyalkyl fluorene derivative (PAF) (for example, refer to Non-Patent Documents 7, 8), and a technique in which a low molecular weight luminescent colorant is dissolved or dispersed in a polymer such as polystyrene, polymethylmethacrylate, or polyvinyl carbazole (PVK) (for example, refer to Patent Document 12 and Non-Patent Document 9).
A copolymer of vinylcarbazole and an iridium complex has been reported to serve as an excellent organic electroluminescent element (for example, refer to Non-Patent Document 10).
(Patent Document 1)                Japanese Patent No. 3093796        
(Patent Document 2)                Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 63-264692        
(Patent Document 3)                JP-A No. 3-255190        
(Patent Document 4)                U.S. Pat. No. 6,097,147        
(Patent Document 5)                Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2002-100476        
(Patent Document 6)                JP-A No. 2001-181616        
(Patent Document 7)                JP-A No. 2001-247859        
(Patent Document 8)                JP-A No. 2002-83684        
(Patent Document 9)                JP-A No. 2002-175884        
(Patent Document 10)                JP-A No. 2002-338588        
(Patent Document 11)                JP-A No. 2003-7469        
(Patent Document 12)                JP-A No. 4-212286        
(Non-Patent Document 1)                M. A. Baldo et al., nature, 395, 151-154 (1998)        
(Non-Patent Document 2)                M. A. Baldo et al., nature, 403(17), 750-753 (2000)        
(Non-Patent Document 3)                S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001)        
(Non-Patent Document 4)                M. E. Tompson et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu)        
(Non-Patent Document 5)                Moon-Jae Youn. 0 g, Tetsuo Tsutsui et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu)        
(Non-Patent Document 6)                Ikai et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu)        
(Non-Patent Document 7)                Nature, 357, 477 (1992)        
(Non-Patent Document 8)                Advanced Materials, Section 4, (1992)        
(Non-Patent Document 9)                Proc. 38th Applied Physics Rengo-Koenkai, 31 p -G-12 (1991)        
(Non-Patent Document 10)                H15, NHK Hoso-giken Kokai-yokoushuu, pp 52-57 (2003)        
However, the organic EL elements obtained so far have not been fully satisfactory in luminance, luminous efficiency, emission life and power consumption.
An object of the present invention is to provide an organic electroluminescent element exhibiting high luminance, high luminous efficiency, long emission life and low power consumption, and to provide a display and an illuminator employing the same.